Ink jet printing has already replaced or supplemented in many fields classic image reproduction methods such as photography on silver halide materials, off-set printing and screen printing. It is used in office surroundings, in the photographic field, in the graphic field, especially in proofing, and, generally, in the production of colored textile materials or in industrial environments. In ink jet printing, extremely small ink droplets are expelled through the nozzles of a printing head and are deposited on a recording medium, using digital image data. The inks used therein consist of solutions or dispersions of colored or black dyes or pigments in a liquid vector. The liquid vector consists, depending on the type of the dye or the pigment, of water, mixtures of water with water-miscible organic solvents, mixtures of exclusively organic solvents, of oils or hydrophobic organic solvents.
Ink jet printing has attained a remarkable degree of maturity. For very demanding applications in the graphic field and the photographic field, however, further improvements are necessary. In these fields, the light stability and the volume of the color space (gamut) are extremely important. Both properties are crucially influenced by the constitution of the dyes or pigments that are used. Pigments normally show a better light stability than soluble dyes. In most cases, however, dyes have a higher brilliance and therefore show more brilliant colors. Furthermore, pigments have a tendency to stay on the surface of the printed medium and therefore to reduce the gloss of the surface. This is not desirable especially for photographic applications of ink jet printing.
Up to now, dye based inks provide a better reproduction of brilliant colors. For applications where a high brilliance of the colors is needed, inks containing dyes and not pigments are preferentially used. Generally, the use of dye-based inks also gives a cost advantage, because in most cases dyes are cheaper than pigments.
In ink sets for demanding applications such as in the graphic field, in photography or in proofing applications, the most brilliant available dyes are used. These dyes must show, besides good water fastness and high diffusion fastness, an excellent light stability.
The simplest ink set consists of one yellow ink, one magenta ink, one cyan ink and one black ink. The three colored inks (primary colors) allow, by superposition, the reproduction of the secondary colors such as red, blue and green and of all intermediate colors.
Inks for ink jet printing need to satisfy stringent requirements. They need to show excellent properties with regard to chemical and physical stability, toxicity and rheology. They are allowed neither to corrode the constituents of the printing heads, nor to clog the nozzles or to form deposits on the heating elements of thermal printing heads. On the recording sheets, however, the inks need to dry quickly and the dyes are, for example, not allowed to diffuse, they need to be light stable and they are not allowed to deteriorate or bleach when the printed images are stored in contaminated air. Furthermore, they are not allowed to reduce the gloss of the recording sheets or to disturb the process of lamination of such printed recording sheets. The choice of suitable dyes for high quality ink jet printing is strongly limited by these requirements. Only a few of all the known dyes more or less fulfill all the requirements of high quality ink jet printing. Probably no dye exists that fulfills all these requirements completely.
Yellow dyes with reddish hue, as described for example in patent application EP 0,755,984, show an excellent light stability as well as an excellent brilliance and are therefore very suitable for the perfect reproduction of skin tones in ink jet printing of photographic quality. Furthermore, these dyes show a very good diffusion fastness.
In graphic applications, as for example in proofing, these yellow dyes with reddish hue are normally not used. Acid Yellow 23 of formula has exactly the
correct hue and is therefore the most suitable and normally used yellow dye in graphic applications, particularly in proofing. This dye has the disadvantages of bad light stability and bad diffusion fastness on nearly all recording sheets, as well on polymer based recording sheets and on nanoporous recording sheets. Only on uncoated paper its diffusion fastness is acceptable.